1. Field of the Invention
The present invention relates to metal scavengers and also to waste-water treatment processes using the metal scavengers.
2. Description of the Related Art
To cope with the ever-increasing contamination problem of rivers, seas and the like due to waste water from factories, more stringent regulations have been introduced to prevent waste water contamination. Current regulations require metals, contained in waste water, to be lowered to their corresponding prescribed concentrations before discharging it as an effluent into the environment. Further, various heavy metals are contained in slag discharged from mines, fly ash occurring upon incineration of waste at waste incineration plants, sludge produced as industrial waste upon neutralizing coagulation and sedimentation or coagulant-assisted sedimentation in the course of treatment of waste water, highly contaminated soil, and the like. Runoff of such heavy metals into underground water, rivers, and the seas has been increasingly causing problems. It is, therefore, a matter of urgent necessity to establish an effective process for the treatment of such heavy metals. Stringent regulations have been imposed for heavy metals harmful to the human body--such as mercury, cadmium, lead, zinc, copper and chromium. As far as waste water is concerned, a variety of processes have been proposed for the removal of metals. Known processes of this sort include ion flotation, ion exchanging, electrolytic floating, electrodialysis and reverse osmosis as well as the neutralizing coagulation and sedimentation in which an alkaline neutralizing agent such as calcium hydroxide or sodium hydroxide is added to convert metals into their hydroxides and the resultant metal hydroxides are coagulated and sedimented by a high molecular coagulant and then removed.
However, ion flotation, ion exchanging, electrolytic floating, electrodialysis and reverse osmosis involve problems in the degree of removal of heavy metals, operability, running cost, etc. Under these circumstances, these are used only for the treatment of certain special waste waters. On the other hand, the neutralizing coagulation and sedimentation process is accompanied by the problem that a sludge of metal hydroxides is formed in a large quantity but dewatering of this sludge is difficult as is its transportation due to the large volume. In addition, it is very difficult to remove heavy metals to levels below their respective effluent standards. Moreover, these sludges involve the problem that they may be dissolved again depending on the manner of disposal, thereby causing secondary contamination.
In contrast, the treatment of waste water by a metal scavenger can overcome the problems described above, so that metal scavengers are widely used in recent years. Of metal scavengers of this sort, there are known metal scavengers, each of which is composed of a polyamine derivative containing one or more dithiocarboxyl groups as functional groups. Various waste water processes making use of such metal scavengers have hence been proposed (for example, Japanese Patent Publication Nos. 39358/1981, 57920/1985 and 3549/1989 and Japanese Patent Application Laid-Open No. 65788/1987).
The above polyamine derivatives have excellent metal-scavenging efficiency and upon scavenging metal ions, form large floc of high sedimentation velocity. They can therefore perform the removal of metal ions from waste water more efficiently than conventional processes. However, adsorption of chromium (III), nickel, cobalt and manganese is still not considered sufficient. A cake obtained by separating and solidifying floc formed as a result of adsorption of metals on metal scavengers is disposed by incineration or by solidification with cement. When conventional metal scavengers are used, the cake has a relatively high water content. This may lead to a problem such that enormous energy is required upon incineration of the cake or the cake has a large volume and requires an unreasonably large amount of labor and money for its processing and disposal.
As a method for the disposal of metals contained in fly ash, sludge, slag or soil, it has heretofore been the practice to use such metal-containing waste for reclamation or landfilling or to dump it in the ocean after solidifying it with cement as is. These methods are not considered safe because there is the potential danger that such metals may run off through the cement layer. Hence, there has been a strong demand for the development of an absolutely safe disposal method.